Sodium-induced aggregation of phosphatidic acid and mixed phospholipid vesicles

Sodium-induced aggregations of sonicated vesicles prepared from synthetic phosphatidic acid and from its 1 : 1 mixtures with synthetic phosphatidylethanolamine and phosphatidylcholine were studied by turbidimetric measurements. The aggregation reactions were almost completely reversible on change in the Na⁺ concentration, pH or temperature. The threshold concentrations of Na⁺ for aggregation of pure dipalmitoylphosphatidic acid vesicles and mixed dipalmitoylphosphatidylenolamine- and dimyristoylphosphatidylcholine-dipalmitoylphosphatidic acid vesicles were found to be 200, 310 and 550 mM, respectively, at 25° and pH 7.2. The hydrocarbon chain lengths of phosphatidic acid and phosphatidylethanolamine had little effect on the threshold concentrations. The threshold concentrations for phospholipid vesicles composed of phosphatidic acid alone or its 1 : 1 mixture with phosphatidylethanolamine were changed by varying either the pH or temperature, while that for phosphatidylcholine-phosphatidic acid vesicles was almost independent of the pH and temperature, implying that aggregation of the latter vesicles is induced by a somewhat different mechanism.